An image sensor is a semiconductor device used to convert optical images detected by the image sensor to electric signals. Image sensors may be classified as a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS).
A CMOS image sensor is provided with a plurality of MOS transistors corresponding to pixels of a semiconductor device having a control circuit and a signal processing circuit as peripheral circuits. The control circuit and the signal processing unit may be integrated together to employ a switching method that detects output through the MOS transistors.
A CMOS image sensor may be provided with a plurality of unit pixels whereby each unit pixel includes one light sensing device such as a photodiode and a plurality of MOS transistors.
As illustrated in example FIG. 1, a CMOS image sensor includes a plurality of photodiodes 22, 24 and 26 formed on and/or over silicon substrate 10 by repeatedly performing ion implantation and a silicon epitaxial growth process. Photodiodes 22, 24 and 26 are configured to detect red light, green light and blue light. An ion implantation layer for red photodiode 22 can be formed in substrate 10 and first silicon epitaxial layer 12 can be formed thereon and/or thereover. Green photodiode 24 can be formed in first epitaxial layer 12 using an ion implantation process. Plug 42 for contacting red photodiode 22 can be formed in a portion of first epitaxial layer 12. Using a silicon epitaxial growth process, second silicon epitaxial layer 14 is formed and blue photodiode 26 is formed in second silicon epitaxial layer 14. Plug 46 for contacting green photodiode 24 and plug 44 for contacting red photodiode 22 can be formed. The plurality of MOS transistors for transferring optical charges detected by photodiodes 22, 24 and 26 can be formed on and/or over second silicon epitaxial layer 14. Each of the plurality of MOS transistors may include gate 30, gate insulating film 32, and spacer 34.
Because the respective sizes of red photodiode 22 and green photodiode 24 are larger than blue photodiode 26, and the transistors for delivering signals are arranged on an uppermost layer, only the size of blue photodiode 26 becomes a substantial light reception area. Consequently, the substantial light reception area can be smaller than that of the photodiodes. Plugs 42, 44 and 46 for processing the signal of red photodiode 22 or green photodiode 24 can be formed by the ion implantation process. However, if light is irradiated, noise may be generated in the signal due to the plugs. Photodiodes 22, 24 and 26 may be separately formed in order to prevent interference between them. An additional ion implantation layer for isolating photodiodes 22, 24 and 26 can be formed in each epitaxial layer 12 and 14.